A large amount of color printing has been practiced in the art by way of off-set printing, gravure printing, etc. In carrying out such color printing, an original manuscript as it is, or combined with another manuscript, letters, symbols, etc., is subjected to color resolution to prepare a plate of the three primary colors of cyan, magenta, yellow, and further a plate of black is added if desired, to reproduce the hue, the pattern, etc., of the manuscript with the respective printing inks.
As the color material of the three primary color inks of cyan, magenta and yellow in such a printing system, pigments have been used in most cases, and these pigments are selected from the most preferable pigments of the three primary colors based on a large number of experiences in the past so that the three primary colors as a matter of course, and the intermediate colors therebetween could be all reproduced broadly.
Since printing systems of the prior art as described above always require indispensably preparation of the plate with the three primary colors or with further addition of black, there arises a problem in that a high expenditure for installation and much space are required. For example, there is a problem in that color printing cannot be performed simply in small factories or offices.
On the other hand, with the development of photographic technology in recent years, color photography has been greatly utilized, but reproduction of these color photographies are not as easy as printing, and there is also the drawback that this reproduction becomes expensive as the size becomes greater.
As one method for solving such problems, a heat transfer system for formation of color image in which a heat transfer sheet of the three primary colors is prepared from sublimatable (or heat migratable) dyes, and the dyes are transferred by heat energy by utilizing this heat transfer sheet to form a color image has been proposed. Such a system, which requires no great printing machine or other various auxiliary equipment and makes possible formation of a color image easily, is expected to be developed in the future.
The above heat transfer system is a method in which a heat transferable material (image receiving sheet) and a heat transfer sheet are superposed on one another, and heat energy is imparted by a printing means such as a thermal head from either side, thereby transferring the dyes on the heat transfer sheets onto the heat transferable sheet, and the size of the color dots formed by this transfer is very much greater than those of the dots in off-set printing of the prior art. Also, in the case of printing ink, the color density of the dot can be freely changed principally by the size of the dot, while in the case of heat transfer sheets the dot size cannot be easily changed, and the difference in density cannot but be changed by the heat energy imparted, but the delicate change in density by this method is very difficult.
From the difference between the two systems as described above, when a color image is to be formed by the heat transfer system, the scope of its color reproducibility is remarkably inferior as compared with the color image formed by off-set printing, etc., and improvement in this respect has been desired.
Also, the colors of the three colors of off-set printing ink of the prior art are constituted mostly of pigments, while the color materials to be used in the heat transfer sheet are all sublimatable (or heat migratable) dyes, and therefore the two are different from each other in their color forming mechanisms, whereby it has been substantially impossible to select heat migratable (sublimatable) dyes coinciding with the three primary colors of off-set printing ink.
Further, in the dyes of the prior art, when a color image is to be formed by use of the three primary colors of cyan, magenta and yellow, reproduction of the intermediate colors between these three colors has been extremely difficult, and for obtaining a color image approximating the printed image in the heat transfer system, it has been an important technical task to develop a heat transfer sheet having broad color reproducibility not only in the three primary colors but also in the intermediate colors therebetween.